1. Field of the Invention
The present invention relates to a sheet feeding device and an image forming apparatus, and more particularly to a pneumatic suction sheet feeding device that separates a sheet located at the top of a stacked bundle of sheets to feed one sheet at a time, and an image forming apparatus such as an electrophotographic copying machine, a facsimile, or a printer including such a sheet feeding device.
2. Description of the Related Art
As widely known, types of sheet feeding devices for taking out and feeding one sheet at a time from a stacked bundle of sheets generally include frictional sheet feeding devices and pneumatic sheet feeding devices. Japanese Patent Application Laid-open No. 2001-31273, for example, discloses an example of a pneumatic sheet feeding device that separates sheets in a bundle by blowing or discharging air thereto, and feeds the sheet downstream in a conveying direction using a sheet feeding unit consisting of various rollers located at a portion of a sheet supply tray near a leading-edge in a feeding direction.
In addition, suggestions have recently been made for a pneumatic suction sheet feeding device including a plurality of air blowing units blowing or discharging air to separate sheets in a bundle more reliably by causing the air blowing units to blow the air to the sheets, and feeding a top sheet thus separated while suctioning the sheet with an air suctioning unit.
As this type of pneumatic suction sheet feeding devices, a sheet feeding device illustrated in FIG. 11 is known.
As illustrated in FIG. 11, a conventional sheet feeding device 200 includes an air blowing unit 202 that blows air to a bundle of sheets stacked on a paper feed tray 201 to float and to separate upper sheets including at least the top sheet in the bundle, an air suctioning unit 203 that suctions the top sheet thus separated, and a sheet feeding unit 204 having a suctioning belt for feeding the sheet thus suctioned and retained by the air suctioning unit 203 in a sheet feeding direction. The air blowing unit 202 includes a blower fan for generating the air to be blown to the sheet bundle, and the air suctioning unit 203 includes a blower fan for generating air for suctioning the top sheet.
The air blowing units 202 are arranged each at the leading-edge of the sheet bundle in the feeding direction and at both side edges of the sheet bundle. These air blowing units 202 are configured to blow the air to the leading-edge of the sheet bundle in the sheet feeding direction and to the side edges of the sheet bundle. The air blowing unit 202 arranged at the leading-edge side of the sheet bundle in the sheet feeding direction includes an air blowing unit for blowing floating air for floating upper sheets including at least the top sheet in the sheet bundle, and an air blowing unit for blowing separating air for separating the sheets.
The paper feed tray 201 can be moved up and down by a lifting unit not illustrated. A sheet position detecting unit 205 is configured to detect the position of the top sheet in the sheet bundle stacked on the paper feed tray 201.
The sheet position detecting unit 205 has an actuator 205a that has a roller located at the tip thereof and abutting on the top sheet and that can be swung depending on the height of the sheet bundle, and a detecting sensor 205b such as a photointerrupter that detects an amount of the swing of the actuator 205a. 
The position to which the paper feed tray 201 is moved up and down is adjusted so that a distance h between the top sheet and the sheet feeding unit 204 is kept constant based on a detection signal of the detecting sensor 205b. 
A sheet blocking member 206 for preventing sheets other than the top sheet from being fed is arranged at a side of the paper feed tray 201 near the leading-edge in the feeding direction.
An operation of the conventional sheet feeding device will now be explained with reference to the flowchart illustrated in FIG. 13, and also with reference to FIGS. 12A to 12D as required.
As illustrated in FIG. 13, when a paper feed command is received from an image forming apparatus body (Step S201), the lifting unit not illustrated lifts the paper feed tray 201 (Step S202). When the sheet position detecting unit 205 detects the top sheet reaching a specified position (YES at Step S203), the lifting unit stops lifting the paper feed tray 201 (Step S204).
The air blowing units 202 then start blowing the floating air, the separating air, and side air, and the air suctioning unit 203 starts suctioning the air at the same time (Steps S205 to S208). In other words, the operations at Steps S205 to S208 are all started simultaneously.
When specified seconds elapse from the time when the processes at Steps S205 to S208 are started (YES at Step S209), a paper feeding operation is started (Step S210).
In the paper feeding operation, as illustrated in FIG. 12A, a driving unit not illustrated drives the sheet feeding unit 204, so that a top sheet P1 that is separated, suctioned to, and retained by the sheet feeding unit 204 in the operations at Steps S205 to S208 is fed towards carriage rollers 207.
The carriage rollers 207 then convey the top sheet P1 further upstream as illustrated in FIG. 12B. As illustrated in FIG. 12C, when a paper feeding sensor 208 detects the leading edge of the top sheet P1, driving the sheet feeding unit 204 is stopped. At this time, the carriage rollers 207 continue to convey the top sheet P1 while the sheet feeding unit 204 is no longer driven.
When the top sheet P1 passes through the suctioning section, a sheet P2 that is a next original is floated, separated, suctioned, and retained as the top sheet in the operations at Steps S205 to S208 as illustrated in FIG. 12D. The driving unit starts driving the sheet feeding unit 204 again based on a preset paper feeding interval to feed the sheet P2.
By repeating these operations, the sheets stacked on the paper feed tray 201 are fed sequentially.
When the abovementioned paper feeding operation is completed (Step S211), the air blowing units 202 stop blowing the floating air, the separating air, and the side air, and the air suctioning unit 203 stops suctioning the air simultaneously as illustrated in FIG. 13 (Steps S212 to S215). In other words, the stopping operations at Steps S212 to S215 are all performed simultaneously.
A series of paper feeding jobs is then ended (Step S216).
A large amount of power is consumed immediately after each of the blower fans in the air blowing units 202 and the air suctioning unit 203 is started, as illustrated in FIG. 14B.
In the conventional sheet feeding device, the air blowing units 202 start blowing the floating air, the separating air, and the side air, and the air suctioning unit 203 starts suctioning the air all at the same time (see Steps S205 to S208 in FIG. 13). Therefore, the moments of surging power consumption coincide immediately after the blower fans in the air blowing units 202 and the air suctioning unit 203 are started, and an enormous amount of power is consumed as a whole as indicated as a rise in a waveform S illustrated in FIG. 14A.
Furthermore, as illustrated in FIG. 15, in the conventional sheet feeding device, because the air is blown simultaneously to the leading edge of the sheet bundle in the paper feeding direction and to the side edges of the sheet bundle, the position of the sheets is not stabilized when they are floated. Therefore, the sheets are fed without having their position stabilized, resulting in a skew or a registration error.
Moreover, in the conventional sheet feeding device, the air blowing units 202 stop blowing the floating air, the separating air, and the side air, and the air suctioning unit 203 stops suctioning the air all at the same time (see Steps S212 to S215 in FIG. 13) when a series of paper feeding jobs is completed. However, the inertia keeps the blower fans of the air blowing units 202 and the air suctioning unit 203 running.
Therefore, even if the paper feeding jobs are completed, sheets are kept suctioned to and retained by the sheet feeding unit 204.
Thus, if the user considers that the paper feeding operation has been completed in this condition and opens the paper feed tray 201, the sheets kept suctioned to and retained by the sheet feeding unit 204 due to the inertia of the blower fans might not return to a predetermined position in the paper feed tray 201, and might fall into a position in the sheet feeding device body where the user would have a difficult time removing them, or the sheets might be damaged due to such causes.